1. Field of the Invention
The invention pertains to the field of electric circuits. More particularly, the invention pertains to apparatus and method for DC/DC converters having high speed and high accuracy. Yet more particularly, this invention pertains to the control of the output voltage of DC/DC converters.
2. Description of Related Art
DC/DC converters are known to be used in various industries. The usage includes power supplies for computers, personal digital assistants, cellular phones and other hand held mobile electronic devices and systems. Each usage may have specific demands. Further, DC/DC converters have various types of output voltage control which utilizes pulse width modulation (PWM) type. In general, PWM type DC/DC converters can be subdivided into analog pulse width modulation control and digital pulse width modulation control.
Analog Pulse Width Modulation Control
Analog pulse width modulation control is by far the most prevalent method for controlling DC/DC Converters. A block diagram of a DC/DC converter with this type of control is shown in FIG. 1. The control circuit blocks, which comprise the conventional analog pulse width modulation control circuit, are shown within the dotted section of FIG. 1. The remainder of the circuitry of FIG. 1 provides the actual DC/DC conversion function.
FIG. 1 shows a transformer isolated forward converter topology for the DC/DC converter function. The same type of control circuit, however, is used for non-isolated forward converter topology, isolated and non-isolated flyback, push pull, half bridge, full bridge, Sepic, Cuk, Weinberg, Severns, and other topologies. Although we will limit our discussion to the Isolated Forward Converter topology of FIG. 1, it should be understood that any other DC/DC converter topologies are controlled in the same manner.
The conventional analog control method regulates the DC Output voltage of a DC/DC converter by varying the ratio of the xe2x80x9conxe2x80x9d time and xe2x80x9coffxe2x80x9d time of the DC/DC Converters power switch Q1. Control of the DC output voltage starts by using an error amplifier (U2) for comparing a sample of the output voltage to a voltage reference. The error amplifier outputs a voltage, which is proportional to the difference between a sample of the DC output and the voltage reference. Since the error amplifier has a very high DC gain (usually more than 10,000), a very small DC output error (less than 1 millivolt) will produce a very large change in error amplifier output voltage. The output of the error amplifier connects to the pulse width modulator circuitry comprised of a comparator U3, ramp generator U4, latch U5, and clock generator U6.
Waveforms associated with the pulse width modulation function are shown in FIG. 2. The pulse starts when the clock generator turns on the latch. This turns on Power switch Q1 through switch driver U1, and starts the flow of power through transformer T1, rectifier CR1, and output filter L1 and C1.
The clock generator also starts a ramp (shown in FIG. 2), which connects to one side of the comparator U3. The other leg of the comparator connects to the output of the error amplifier U2. The comparator U3 changes state and turns off the latch U5 when the ramp generator U4 output becomes the same as the error amplifier output, which in turn shuts off transistor Q1. The process repeats when the clock generator U6 turns on the latch U5 again.
The pulse width generated by the above control circuit depends on the output of the error amplifier U2. If the DC output is too low, the error amplifier output increases, thereby increasing the pulse width driving transistor switch Q1, which increases the DC output. Conversely, if the DC output is too high, the error amplifier voltage decreases, thereby decreasing the pulse width driving transistor switch Q1, and decreasing the DC output. The DC output is thus accurately regulated by the action of the error amplifier U2.
The DC output not only needs to be accurately regulated, it also needs to be stable. DC outputs are known to possess an AC ripple component. Stability means that the DC output has an AC ripple component at the same frequency as the clock generator U6. In order for this to occur, for a fixed DC input voltage and for a fixed DC output load, the pulse width driving power switch Q1 must not change from pulse to pulse,. This is shown in FIG. 2 as a constant error amplifier output, which generates the same pulse width among the three pulses that are depicted. It is pointed out that those skilled in the state of the art should know how to choose stabilization components R1, C2, C4, R3, and C3 so as to stabilize the DC output for a given clock generator frequency and output filter components L1 and C1.
Also of importance in DC output stabilization is the choice of ramp renerator U4. If the ramp is fixed and unchanging, it is called voltage mode control. On the other hand, if the ramp is derived from the inductor current L1, it is called current mode control. The choice of stabilization components is different for these two types of ramp generators.
Analog PWM control methods for DC/DC converter have their drawbacks. One of them is the byproducts of stabilizing the DC output in that the error amplifier is slowed down. The DC output, therefore, is limited by the speed with which it can respond to a change in DC input voltage or DC output load current.
FIG. 3A shows a typical response of the DC output voltage to a change in DC output load current. As the DC output load steps from one value to a higher value, the DC output voltage at first drops. The error amplifier eventually responds and corrects for this drop. Likewise, as shown in FIG. 3B, when the DC output load current steps from a higher value to a lower value, the DC output voltage first goes up, before the error amplifier responds and corrects for this increase. The speed with which the DC Output voltage corrects is called the transient response. The transient response is a complex combination of clock generator frequency, choice of output filter components, and choice of stabilization components. As a general rule, however, a well-stabilized DC/DC converter output cannot respond any faster than 50 to 100 clock generator cycles. This then is the limiting factor in the speed in which an analog pulse width modulated DC/DC converter can respond to changes in DC Input voltage or changes in DC output load current.
Digital Pulse Width Modulation Control
By digital pulse width modulation control, it is generally referred to the utilization of a voltage comparator without an error amplifier for the DC output voltage control of a DC/DC converter. Digital regulation started in the early days of transistor DC/DC switching regulators, going back to the 1970""s. These regulators were called ripple regulators. A single comparator turned the transistor switch on and off based on the DC output voltage ripple. There was no clock generator or latch. The delays through the various circuit components determined the frequency of the regulators"" operation.
The technique became more sophisticated, and resulted in a patent being granted in 1993 to Harry E. Wert (see U.S. Pat. No. 5,260,861). An equivalent block diagram of the DC output voltage control portion of the Wert patent is shown in FIG. 4. Ac can be seen, the control of the DC output voltage starts by comparing a sample of the DC output voltage to a voltage reference by a voltage comparator, U2.
Waveforms associated with the pulse width modulation function are shown in FIG. 5. The pulse starts when the clock generator U4 turns on the latch U3. This turns on Power switch Q1 through switch driver U1, and starts the flow of power through transformer T1, rectifier CR1, and output filter L1 and C1.
The comparator U2 changes state and turns off the latch U3 when the sampled DC output becomes the same as the Voltage reference U5, which in turn shuts off transistor Q1. The process repeats when the clock generator turns on the latch again.
The pulse width generated by the above circuit depends on the DC output (Vout). If the DC output is too low, the comparator output stays in a high state longer, increasing the pulse width driving transistor switch Q1, which increases the DC output. Conversely, if the DC output is too high, the comparator output stays high for a shorter period of time, thereby decreasing the pulse width driving transistor switch Q1, and decreasing the DC output. The DC output is thus regulated by the action of the voltage comparator U2.
As with the previous discussion of analog pulse width modulation, the digital pulse width modulation control also needs to be stable. Stability means that the DC output has an AC ripple component at the same frequency as the clock generator. In order for this to occur, for a fixed DC input voltage and a fixed DC output load, the pulse width driving power switch Q1 must not change from pulse to pulse. This is shown in FIG. 5 as a constant ripple component of the DC output sample (Vout), which generates the same pulse width among the three pulses shown.
It is pointed out that in order for the DC output to be stable, the ripple component of the DC output sample must have a large enough slope to act as the Ramp generator acts in the analog pulse width modulation method. This is one of the drawbacks of the digital control method since the DC output ripple must be much higher than that of the analog control, and the ripple amplitude must be accurately controlled. Controlling the amplitude of the ripple is difficult because it is a function of a parameter of capacitor C1 called the Equivalent Series Resistance, or ESR. The ESR of capacitors varies widely from unit to unit, thereby causing difficulty in maintaining a stable DC output in a production environment. The difficulty in maintaining stable DC output is one of the reasons why the digital control method is generally not used for the control of high performance DC/DC Converters.
Because of its simplicity, however, the digital control method is used for the control of low performance, low cost DC/DC converters. For these applications, instability, which causes a low frequency component to show up in the DC output, and thus requires a much larger output filter to be used, may not matter much. Oscilloscope photographs of a DC output with this type of low frequency instability are shown in FIGS. 6A and 6B.
The digital control method suffers from a second drawback in that the output voltage is not regulated as accurately as with the analog control method. This is because there is no high gain error amplifier, which produces a large voltage change due to a very small error in DC output. This phenomenon can be inferred from the Digital Pulse Width Modulator waveforms of FIG. 5. As can be seen, during the latch output xe2x80x9conxe2x80x9d time, the sampled DC output (Vout) ramps up until it touches the Voltage Reference (Vref), at which point the latch output turns off and the sampled DC output begins to ramp down. If the DC Input voltage or the load output current change in such a way that a much smaller pulse width is needed to maintain output voltage regulation as shown in the last 3 pulses of FIG. 5, the ramp-up portion of the DC output becomes smaller and the ramp-down portion becomes larger. This forces the average sampled DC output voltage to rise, thereby causing the DC output voltage to increase. Likewise, if a larger pulse width is needed due to a change in DC Input voltage or output load current, the ramp-up portion of the sampled DC output voltage becomes larger, while the ramp-down portion becomes smaller. This causes the DC output voltage to drop.
This less than perfect regulation of the DC output voltage may be of little consequence for low performance, low cost DC/DC converter applications. For high performance DC/DC converters, however, a much tighter output voltage regulation is generally required.
One advantage of digital control method is the speed with which it reacts to changes in DC Input voltage or changes in output load current. While the analog control method requires 50 to 100 clock cycles to react to a transient condition, the digital control method typically merely requires 10 clock cycles or less.
FIG. 7 shows a typical response of the DC Output voltage utilizing digital control, to a change in DC output load current. As the DC output load steps from one value to a higher value, the DC output voltage drops. The comparator corrects for this drop but not completely. As can be seen, there is a difference between the output voltage prior to the load change, and after the load change. The speed with which the DC Output voltage corrects, i.e. the transient response, is measured to be approximately 5 to 10 clock cycles.
To summarize the above discussion, the plusses and minuses of the analog control method are as follows. The plusses are: most prevalent and most well understood (a plus); generates a highly accurate DC output voltage (a plus); well-understood stabilization procedures to those skilled in the art, (a plus); and low output voltage ripple (plus).
However, one of the significant drawbacks of the analog control method is that the method is slow to respond to changes in DC input voltage or to changes in output load. The response typically requires 50 to 100 clock cycles (a significant minus).
On the other hand, the plusses and minuses of the digital control method are as follows. The plusses for digital method includes low cost on an unit basis, and simple to implement, as well as very fast response to changes in DC input voltage or output load (plusses).
However, digital control method has a number of minuses. The minuses include low performance levels, having stabilization dependent on parameters that cannot be well controlled (a minus); high output ripple being required in order to maintain stability (a minus); and DC output voltage being not very accurately controlled (a minus).
As can be appreciated, although prior art conventional analog pulse width modulation control methods for DC/DC Converters provide for a high accuracy output voltage regulation having low output voltage ripple, but the methods are slow to respond to changes in load current or input voltage. On the other hand, prior art conventional digital control pulse width modulation control methods provide for high-speed response to changes in output load current or input voltage, but this provision is achieved at the expense of poor accuracy in output voltage regulation, and in higher output voltage ripple.
Therefore, it is desirous to have a circuit and method which blends the digital and analog control methods of DC/DC converters so as to obtain a DC/DC Converter which possesses the speed of the digital control method and the output voltage accuracy and low output voltage ripple content of the analog control method.
A digital control circuit is provided as part of a DC/DC converter to generate a highly accurate output such as a DC output.
A digital control circuit is provided as part of a DC/DC converter to generate a highly accurate DC output with low output voltage ripple.
As part of a DC/DC converter system and method, a digital control circuit that is fast in responding to changes in DC input voltage is provided.
As part of a DC/DC converter system and method, a digital control circuit that is fast in responding to changes in output load is provided.
As part of a DC/DC converter system and method, a digital control circuit that can be implemented at low cost is provided.
As part of a DC/DC converter system and method, a digital control circuit that can be better controlled than prior art circuits is provided.
As part of a DC/DC converter system and method, a digital control circuit that can be stabilized independent of parameters that cannot be well controlled.
A DC/DC converter system and method that does not require high output voltage ripple to maintain stability are provided.
A DC/DC converter system and method that accurately controls DC output voltage are provided.
A DC/DC converter system and method that possesses fast response to changes in DC input voltage is provided
A DC/DC converter system and method that possesses fast response to changes in output load is provided.
Accordingly, a digital pulse width modulator (PWM) control circuit which is coupled to the DC output is provided. The circuit includes a first input being a sample of the DC output; a second input being coupled to the DC output and possessing a ripple signal in synchronization with a DC output component; a voltage reference for determining a desired DC output; an adjustment amplifier having a portion of the DC output and the voltage reference as inputs, thereby amplifying a first difference; and a comparator having the first input and the output of the adjustment amplifier as inputs thereto with the second input overlaying one of the comparator inputs, the comparator comparing a second difference. Thereby a determination is made as to whether the DC output is above or below the desired DC output and a correction is performed to change the DC output to a set of values as close to the desired DC output as possible.
Accordingly, a DC/DC converting system is provided that includes a DC input power source disposed to be switched on and off periodically by a first power switch; and at least one DC output receiving power from the DC input power source. The system further includes at least one digital pulse width modulator (PWM) control circuit which is coupled to the DC output. The circuit includes a first input being a sample of the DC output; a second input being coupled to the DC output and possessing a ripple signal in synchronization with a DC output component; a voltage reference for determining a desired DC output; an adjustment amplifier having a portion of the DC output and the voltage reference as inputs, thereby amplifying a first difference; and a comparator having the first input and the output of the adjustment amplifier as inputs thereto with the second input overlaying one of the comparator inputs, the comparator comparing a second difference. Thereby a determination is made as to whether the DC output is above or below the desired DC output and a correction is performed to change the DC output to a set of values as close to the desired DC output as possible.
Accordingly, a method for controlling a DC/DC converter is provided. The method includes the steps of providing a switchable DC input to generate at least one DC output; providing a voltage reference; amplifying a first difference between a portion of the DC output and the voltage reference; and comparing a second difference between a portion of the DC output and the amplified first difference.